Cryostat arrangement comprising a neck tube having a supporting structure and an outer tube surrounding the supporting structure to reduce the cryogen consumption

ABSTRACT

A cryostat arrangement (1) with a vacuum tank (2) and a cryogenic tank (3) are provided. The vacuum tank has at least one neck tube, (4) leading to the cryogenic tank, with a supporting structure (4a) and an outer tube (4b) surrounding the supporting structure. The neck tube provides a connection from the cryogenic tank to a region outside the vacuum tank to allow cryogenic fluid to flow from the cryogenic tank into a region outside the vacuum tank or vice versa. The neck tube mechanically suspends the cryogenic tank inside the vacuum tank, and parts of the neck tube form a diffusion barrier between the interior of the cryogenic tank and the interior of the vacuum tank. The neck tube can connect to other components of the cryostat arrangement in a fluid-tight manner. Heat input from the neck tubes into the cryogenic tank can be considerably reduced thereby.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims foreign priority under 35 U.S.C. § 119(a)-(d) to German Application No. 10 2017 205 279.1 filed on Mar. 29, 2017, the entire contents of which are hereby incorporated into the present application by reference.

FIELD OF THE INVENTION

An aspect of the invention relates to a cryostat arrangement, comprising a vacuum tank and a cryogenic tank, which is arranged inside the vacuum tank, the vacuum tank having at least one neck tube having a supporting structure and an outer tube surrounding the supporting structure, the neck tube leading to the cryogenic tank and, wherein the neck tube produces a spatial connection of an internal volume of the cryogenic tank to a region outside the vacuum tank so that cryogenic fluid can flow out of the cryogenic tank into a region outside the vacuum tank or vice versa.

BACKGROUND

Aspects of the invention generally relate to the area of cooling technical systems which should/must be kept at very low (=cryogenic) temperatures during operation. Such systems can include, for example, superconducting magnet arrangements of the type used in the field of magnetic resonance, such as in MRI topographies or NMR spectrometers. Superconducting magnet arrangements of this type are conventionally cooled by liquid helium as a cryogenic fluid.

An important feature of a superconducting NMR magnet system is the helium consumption during operation. First, helium consumption has effects on costs incurred for the operation of the system; second, the refilling interval is crucially dependent on the helium consumption. The shorter the refilling interval, the longer the system can be operated without errors. In the case of a constant refilling interval, a system having less helium consumption can also have a more compact design, since the helium tank can be smaller. The consequence of this is that the system is cheaper to produce, and the requirements at the installation site (e.g., room height) are reduced. One of the development aims for superconducting magnet systems is thus to reduce the consumption of liquid helium, which, in the case of bath-cooled systems, is equivalent to a reduction in the thermal load on the helium tank.

In typical bath cryostats, a majority of the overall thermal load on the helium tank is caused by thermal conduction in the so-called “neck tubes”. These neck tubes connect the tank, in which the liquid cryogen is stored, to the environment. The cryogenic liquid can be refilled through the neck tubes and can then flow off (also at a high flow rate, such as during a magnet quench or a sudden loss of vacuum insulation). The neck tubes are also necessary for accessing components located in the tank (e.g., the electrical connections of a magnet coil). In many cryostats, the neck tubes also support the weight of the tank. One of the greatest contributing factors to the overall thermal load on the helium tank originates from said neck tubes, the thermal conduction in the tube being the dominant mechanism.

Conventionally, neck tubes have particularly thin walls; wall thicknesses in the range of a few tenths of a millimeter are not uncommon. The neck tubes are typically produced from a material having low thermal conductivity. The wall of the neck tube must not be permeable to gas so that the vacuum insulation of the cryostat does not become contaminated and thus unusable. In addition, the neck tube material must be suitable in terms of connection technology (weldability, solderability). In many cases, stainless steel is used.

U.S. Pat. No. 5,220,800 discloses a generic cryostat arrangement for a NMR magnet system comprising a superconducting magnet coil. The cryostat arrangement comprises a double-walled neck tube, through the annular gap of which helium can flow (see, for example, FIG. 4 in said document).

For the mechanical construction and for fastening outer and inner tubes, U.S. Pat. No. 5,220,800 states that:

“The manner in which the chambers 1 and 2, radiation shields 21 and 22, and cooling tank 23 are suspended in the cryostat 4 on suspension tubes 30 is depicted only schematically in FIG. 1. The connecting elements used are thin-walled tubes or bundles of three centering rods 26 each, a few millimeters in diameter, which have extremely low thermal conductivity and high tensile strength.”

The mechanical suspension of the neck tube and the provision of a fluidic diffusion barrier must thus be physically together in this case, in particular formed using the same material. Separate optimizations of the purely mechanical function of a suspension and the fluidic function of a diffusion barrier, for example, with respect to the selection of material according to type and strength, are therefore neither possible nor envisaged according to the teaching of U.S. Pat. No. 5,220,800.

SUMMARY

An aspect of the present invention reduces the heat input originating from the neck tubes into the cryogenic tank—generally a helium tank—for a cryostat(s) as described herein.

For a cryostat arrangement of the type(s) as described herein, the parts of the neck tube are used to mechanically suspend the cryogenic tank inside the vacuum tank, and the parts of the neck tube are used to construct a diffusion barrier between the interior of the cryogenic tank and the interior of the vacuum tank are arranged so as to be spatially separated from one another and are produced from materials which are optimized independently of one another. The supporting structure supports the weight of the cryogenic tank and is produced from a material for which the ratio σ/θ of a maximum permissible mechanical stress σ, where σ>100 MPa, to θ, which is the integral of the thermal conductivity λ over the temperature range ΔT between 300 K and 4 K, where θ<300 W/m, the following applies: σ/θ>⅓(MPa·m)/W. The outer tube is produced from a material through which cryogenic fluid cannot diffuse, or through which only an unmeasurable amount of cryogenic fluid can diffuse in practice, and which tube can be connected to other components of the cryostat arrangement in a fluid-tight manner so that the resulting integral leakage rate out of the cryogenic tank into the vacuum tank is less than 10⁻⁶ mbar˜l/s.

An aspect of the present invention comprises forming the generally double-walled neck tube in such a way that the functions of mechanically fastening the tank and producing the fluid-tight connection (minimal permeation of the cryogen through the neck-tube wall) are separate from one another. This approach allows each function to be optimized independently of the other.

However, the reduction in the thermal load which is achievable by an aspect of the present invention is of great relevance not only for superconducting magnet systems. For this reason, an aspect of the invention is also beneficially applicable to other areas of cryogenics (e.g., storing cryogens such as helium or hydrogen).

According to an aspect of the present invention, in particular, the following advantages are achieved:

The thermal load on the cryogenic tank can be greatly reduced by the design of the neck tube which is more efficient with respect to its thermal properties.

In the case of actively cooled systems, this allows for the use of a cooler having a lower cooling capacity, which has, for example, an advantageous effect on power consumption and system costs.

In the case of bath-cooled systems, this leads to a reduction in the evaporation rate of the cryogenic fluid. First, this results in a considerable reduction in the operating costs, and second, the time interval in which the cryogenic fluid (typically helium) has to be refilled also increases, which reduces disruptions in long-lasting nuclear magnetic resonance measurements and increases the availability of the system for NMR measurements overall.

Evaporating cryogen is in thermal contact with parts of the neck tube. This makes it possible to use the enthalpy of the cold gas to “absorb” heat which flows in the neck tube from the warm end to the cold end using thermal conduction. In the prior art, corresponding non-fluid-tight supporting structures are always in vacuum, and as a result, these structures lack the feature of thermodynamically advantageous exhaust-gas cooling.

Most preferred is an embodiment of the cryostat arrangement, according to an aspect of the invention, in which the supporting structure is in the form of an inner tube, and wherein the inner tube produces a spatial connection of the internal volume of the cryogenic tank to a region outside the vacuum tank so that cryogenic fluid can flow out of the cryogenic tank into a region outside the vacuum tank or vice versa. Using the inner tube, access to the cryogenic tank can be achieved, e.g., to provide electrical connections to a superconducting magnet coil. In addition, a sufficiently large cross section is available to allow cryogen having a high flow rate out of the helium tank to leak out of the cryogenic tank without an excessive increase in pressure (e.g., in the case of a quench of a superconducting magnet coil or in the case of a loss of vacuum).

In an embodiment, the outer tube is in direct, preferably thermally well-conducting, contact with the inner tube. The diffusion barrier can be applied directly to the supporting tube. The inner tube and outer tube are thus in good thermal contact with one another. Cryogen, which flows off through the inner tube, cools the inner tube first and foremost but, since the tubes are in good thermal contact, is also able to absorb heat from the outer tube. The greatest advantages of this embodiment lie in the simplicity of the construction and the thermodynamic efficiency.

In an alternative embodiment, the outer pipe can be at a distance from the supporting structure, and a gap can remain open between the inner tube and the outer tube. This gap can be used in various ways, as described herein. It is thus possible, for example, to let cryogen flow through the gap between the supporting structure and the outer tube, which allows particularly efficient cooling of the two tube walls. In addition, it is possible to use the gap as a pump line for operating a Joule-Thomson (JT) cooler.

If, in this embodiment, the supporting structure is in the form of an inner tube which is closed during normal operation, the advantages already mentioned above (e.g., a large cross section for electrical connections and quenches/loss of vacuum) can be achieved, while the “exhaust-gas cooling” of the neck tube can simultaneously be optimized by a corresponding selection of the gap geometry (e.g., small gap dimensions for high heat-transfer coefficients).

More preferred are variants of this development in which the outer tube and the supporting structure are interconnected by a plurality of axially arranged, radially extending thermal bridges. As a result, even when the supporting structure is at a distance from the outer tube, good thermal contact between the two tubes and fluid flowing in the inner tube or gap can be ensured. Thermal bridges of this type can be, for example, beryllium copper springs which are fixed to the supporting structure in a thermally conductive manner. If the supporting structure is then pushed into the outer tube during system assembly, the beryllium copper springs press against the inner face of the outer tube, by which good thermal transfer can be achieved.

In further advantageous embodiments, the gap between an inner tube and the outer tube comprises a flow restrictor at the end of the neck tube which is closer to the cryogenic tank. If the inlet of the gap is protected by a restrictor, and the room-temperature-side outlet of the gap has sufficiently large dimensions, the quench pressure does not have to be taken into consideration when dimensioning the outer tube, since high pressure cannot build up in the gap. This makes it possible to form the outer tube, which tends to be produced from a material having a relatively great thermal conductivity (e.g., stainless steel) in order to achieve a sufficiently fluid-tight connection, with particularly thin walls, which in turn minimizes the axial thermal conduction in the outer tube.

The inner tube will typically have a wall thickness of between 0.5 mm and 3 mm. In order to minimize the heat input into the cryogenic tank, the wall thickness should be as thin as possible—in particular, as thin as the mechanical strength requirements allow. Magnet coils of a size similar to those referenced herein result in a range of wall thicknesses as indicated above, for typical neck tube diameters and using the materials mentioned further below.

In further preferred embodiments of the invention, the supporting structure is produced from plastics material, preferably from fiber-reinforced plastics material, in particular from glass-fiber reinforced plastic (GRP), more preferably from the fiber-reinforced composite G10. In the case of GRP, the minimum achievable wall thickness is restricted by manufacturing limitations. GRP cannot be made as thin as desired. G10 is a popular material in cryogenics which is characterized by a particularly low ratio of thermal conductivity to strength. G10 is therefore ideally suited to fastening structures having low thermal conductivity. In addition, G10 is relatively cheap and can be molded into numerous shapes. Metal connection pieces can be connected in a simple manner to G10 components by adhesive bonding or, even better, can be laminated directly during the production of the G10 component. When using fiber-reinforced composites, it is possible to orientate the fibers within the matrix in such a way that the anisotropic properties of the fibers are optimally utilized, and e.g., the tensile strength of a support tube in the axial direction is maximized.

In further advantageous embodiments of the invention, the supporting structure made of plastics material comprises a metal extension, preferably made of stainless steel, at each of its two ends. Typically, the inner tubes are connected by metal parts in the cryostat (e.g., to the vacuum tank or the cryogenic tank). The assembly of the cryostat is particularly simple when the plastics pipe is already equipped with metal sleeves at its ends. In that case, no metal-plastics composite has to be produced during the assembly of the cryostat. This is advantageous, since reliable metal-plastics composites are technologically difficult to produce during the assembly process but can easily be integrated directly into the production of the supporting structure made of plastics material (e.g., by directly embedding the metal sleeve in plastics material) or can readily be produced (e.g., by adhesion) in a separate work process before the assembly of the cryostat. It is easy to carry out welding using metal sleeves at the tube ends. Welding is a process which can easily be integrated in the assembly process of a cryostat.

More preferred are developments of these embodiments in which the metal extensions each have a length of between 20 mm and 100 mm, preferably approximately 50 mm, and a cross-sectional area of stress of between 50 mm² and 500 mm². As already mentioned above, the metal sleeves are typically connected by other metal parts in the cryostat (e.g., to the vacuum tank or the cryogenic tank) using welding. The heat input required during welding could damage the plastics tube if there is insufficient distance between the plastics tube and the weld. The dimensions indicated above ensure that the plastics tube is not heated excessively when welding in the metal sleeve.

Another advantageous embodiment of the cryostat arrangement according to an aspect of the invention is that the outer tube is produced from metal, preferably from stainless steel. Metals can be used as a particularly effective diffusion barrier for cryogens. In addition, it is simple to produce reliable and fluid-tight connections between metals (e.g. by welding, brazing or soldering). Stainless steel is one of the most popular materials in cryostat construction. Its low thermal conductivity and good weldability make it ideal for the embodiments described here. In addition, very thin-walled stainless steel tubes having wall thicknesses of a few tenths of a millimeter are readily commercially available and can also be produced cheaply by rolling and longitudinal welding. For cryostats of NMR magnet systems, the beneficial magnetic properties and the low electrical conductivity are also advantageous. The thermal conduction within the diffusion barrier can thus be further reduced (in addition to the favorable material selection).

In the case of a preferred category of embodiments of the invention, inside the neck tube, in particular inside an inner tube and/or between the supporting structure and the outer tube, so-called baffles are installed, which absorb thermal radiation and prevent convection. For the operation of the magnet system, in several respects, it is advantageous for the inner tube to have a large diameter. This makes it easier for example to introduce components, such as power supply lines, signal lines, valve rods, etc., into the cryogenic tank. As the diameter of the inner tube increases, however, the thermal load on the cryogenic tank also increases—first, due to the greater cross-sectional area which is available for transporting thermal radiation and for thermal conduction in the gas column, and second, due to a more favorable geometry for forming convection eddies and thermoacoustic oscillations (Taconis oscillations). If baffles are installed in the neck tube, the thermal radiation is substantially isolated (the baffles act as gas-cooled radiation shields), and the formation of large convection eddies and thermoacoustic oscillations inside the inner tube is prevented. In so doing, the baffles prevent the mass flow. The thermal conduction in the gas column is reduced, since heat transfer resistances occur at each baffle.

More preferred variants of this category of embodiments include foldable baffles. A significant advantage of a large inner tube diameter consists in the fact that, in the case of a large thermal load on the cryogen (e.g., due to a break in the insulation vacuum, in a superconducting magnet system, e.g., by a quench), high pressure cannot build up in the cryogenic tank, since sufficiently large flow-off cross sections are available. If the free cross section of the inner tube is minimized by baffles, however, this advantage is lost. Therefore, it is particularly beneficial for the baffles to be foldable so that, as soon as a large pressure increase, and thus a large mass flow from the cryogenic tank into the region outside the vacuum tank, occurs, the baffles are folded upwards by the out-flowing gas and release the pressure via the cross section of the inner tube.

Also advantageous are embodiments of the cryostat arrangement according to an aspect of the invention comprising a tubular supporting structure, in which the upper end of the inner tube is closed in a fluid-tight manner in normal operation, in particular by a pressure relief valve or a rupture disk, so that cryogenic fluid flowing away in normal operation has to flow through the gap between the inner tube and the outer tube. Cold gas which is produced by the evaporation of the liquid cryogen in the cryogenic tank can still provide considerable cooling performance in the temperature range between the boiling point of the cryogen and the temperature of the vacuum tank. During flow off, the cold gas sweeps along the tube walls and absorbs heat which flows using thermal conduction inside the tube walls of the vacuum tank into the cryogenic tank (“counter-current cooling”). If the cold gas is conducted through the gap between the inner tube and the outer tube, it comes into thermal contact with both the inner tube and the outer tube, by which particularly efficient counter-current cooling can be provided. By selecting a suitable gap geometry, a good balance between a good thermal transfer of the fluid with the walls of the gap and the loss of pressure in the flowing fluid can be achieved.

Further advantageous embodiments of the invention are characterized in that the cryostat contains a JT cooler, in which cryogen is depressurized using a pump located outside the vacuum tank, and in that the gap between the supporting structure and the outer tube is part of the connecting line between the JT cooler and the pump. In turn, the above-described advantage can be utilized in that the cold gas flow off out of the cryostat is used efficiently to reduce the thermal load using thermal conduction. A JT cooler can thus be integrated in a cryostat in a particularly simple manner, since it is not necessary to provide a separate pump line. Absolute fluid-tightness between the pump line (annular gap) and the cryogenic tank (or the volume in the inner neck tube) is not necessary. A low leakage flow is acceptable if it is low by comparison with the flow which is pumped out by the refrigerator.

Furthermore, in embodiments of the cryostat arrangement according to an aspect of the invention, at least one bellows portion can be present in the outer tube so that the outer tube does not absorb any axial forces. If the inner tube and the outer tube are produced from different materials, it is very likely that these two materials have different coefficients of thermal expansion. If the cryostat is cooled down, large mechanical stresses would therefore be produced in the neck tube arrangement. These stresses can be counteracted if a bellows is installed in the outer tube. Said bellows ensures that the tube remains substantially free from stress.

Most preferred are variants of the invention in which the cryostat arrangement is part of an apparatus for nuclear magnetic resonance, in particular for magnetic resonance imaging (=MRI) or for magnetic resonance spectroscopy (=NMR), which preferably comprises a superconducting magnet arrangement. Superconducting magnets for MRI or NMR are conventionally cooled by liquid helium. However, the availability of helium and its price are an essential factor for minimizing helium losses.

Further advantages of aspects of the invention can be found in the description and the drawings. Likewise, the features mentioned above and set out in the following, according to aspects of the invention, can each be used individually per se or together in any combinations. The embodiments shown and described are not to be understood as a definitive list, but rather are in fact examples for describing aspects of the invention.

DESCRIPTION OF THE DRAWINGS

Aspects of the invention are shown in the drawings and described with reference to exemplary embodiments. In the drawings:

FIG. 1 is a schematic vertical sectional view of a first embodiment of the cryostat arrangement according to an aspect of the invention.

FIG. 2 is a schematic vertical sectional view of a second embodiment of the cryostat arrangement comprising baffles in the neck tube according to an aspect of the invention.

FIG. 3 is a schematic vertical sectional view of a third embodiment of the cryostat arrangement comprising a JT cooler and a thermal barrier in the cryogenic tank according to an aspect of the invention.

FIG. 4 is a schematic vertical sectional view of a fourth embodiment of the cryostat arrangement comprising a bellows portion in the neck tube according to an aspect of the invention.

FIG. 5A is a graph of the thermal conductivity integral of stainless steel versus temperature.

FIG. 5B is a graph of the thermal conductivity integral of G10 versus temperature.

DETAILED DESCRIPTION

FIGS. 1 to 4 of the drawings each show, in a schematic view, embodiments of the cryostat arrangement according to an aspect of the invention for storing a cryogen fluid, in particular, for cooling a superconducting magnet arrangement.

A cryostat arrangement 1 according to an aspect of the invention comprises a vacuum tank 2 and a cryogenic tank 3, which is arranged inside the vacuum tank 2, the vacuum tank 2 comprising at least one neck tube 4 having a supporting structure 4 a and an outer tube 4 b surrounding the supporting structure 4 a, the neck tube 4 leading to the cryogenic tank 3 and, wherein the neck tube 4 produces a spatial connection of an internal volume of the cryogenic tank 3 to a region outside the vacuum tank 2 so that cryogenic fluid can flow out of the cryogenic tank 3 into a region outside the vacuum tank 2 or vice versa (from a region outside the vacuum tank 2 into the cryogenic tank 3).

The cryostat arrangement 1 according to an aspect of the invention is characterized, in that, firstly the parts of the neck tube 4 used to mechanically suspend the cryogenic tank 3 within the vacuum tank 2, and secondly the parts of the neck tube 4 used to construct a diffusion barrier between the interior of the cryogenic tank 3 and the interior of the vacuum tank 2 are arranged so as to be spatially separated from one another and are produced from materials which are optimized differently in each case, in that the supporting structure 4 a supports the weight of the cryogenic tank 3 and is produced from a material in the case of which, for the ratio σ/θ of a maximum permissible mechanical stress σ, where σ>100 MPa, to θ, where θ is the integral of the thermal conductivity λ over the temperature range ΔT between 300 K and 4 K, where θ<300 W/m, the following applies: σ/θ>⅓ (MPa·m)/W, and in that the outer tube 4 b is produced from a material through which cryogenic fluid cannot diffuse, or through which only an unmeasurable amount of cryogenic fluid can diffuse in operation, and in which the neck tube can be connected to other components of the cryostat arrangement 1 in a fluid-tight manner so that the resulting integral leakage rate out of the cryogenic tank 3 into the vacuum tank 2 is less than 10⁻⁶ mbar·l/s.

In the embodiments of the invention shown in FIGS. 1 to 4 of the drawings, the supporting structure 4 a is in the form of an inner tube, and wherein the inner tube produces a spatial connection of the internal volume of the cryogenic tank 3 to a region outside the vacuum tank 2 so that cryogenic fluid can flow out of the cryogenic tank 3 into a region outside the vacuum tank 2 or vice versa. In this case, the outer tube 4 b can be in direct, preferably thermally well-conducting, contact with the inner tube—but this is not shown in the drawings. Alternatively, as shown in FIG. 1-4, the outer tube 4 b can be at a distance from the inner tube, and a gap 4 c can remain open between the inner tube and the outer tube 4 b. In this case, the outer tube 4 b and the inner tube are preferably interconnected by a plurality of axially arranged, radially extending thermal bridges.

As can be seen in FIGS. 1, 2 and 4, flow off cryogen may pass through the annular gap 4 c between the inner tube 4 a and the outer tube 4 b in order to optimally use the enthalpy of the cold gas for absorbing the heat which flows from the outer tank (e.g., vacuum tank 2) along the neck tube 4 into the cryogenic tank 3.

The outer tube 4 b produces a fluid-tight connection to the insulation vacuum. The wall thickness is designed in such a way that the maximum differential pressure between the annular gap 4 c and the insulation vacuum can be absorbed, and no significant diffusion of the cryogen into the insulation vacuum takes place. The material is selected in such a way that a fluid-tight connection to other parts of the cryostat (e.g., the cover plate of the cryogenic tank 3) can be produced reliably and cheaply. The outer tube 4 b can be produced e.g., from stainless steel which has excellent welding properties. The wall thickness of said tube can be selected so as to be very thin, since the tube does not have to accommodate the entire weight of the cryogenic tank 3 (and the components located in it).

The inner tube 4 a supports the weight of the cryogenic tank 3. However, it does not have to be hermetically sealed, as a result of which a material can be selected which is primarily characterized by the high ratio of mechanical strength and thermal conductivity. In this case, for example, fiber-reinforced plastics materials are considered. Thus, for example, the supporting structure 4 a can be produced in particular from GRP, more preferably from the fiber-reinforced composite G10.

The GRP tube is connected to a stainless-steel sleeve at its two ends. Connection options between GRP and a stainless-steel sleeve are known to a person skilled in the art. The stainless-steel sleeve must have a certain minimum length (typically 50 mm).

The drawings in FIGS. 5A and 5B show the thermal conductivity integrals for stainless steel (FIG. 5A) and for the fiber-reinforced composite G10 (FIG. 5B). As can be seen, the thermal conductivity integral of stainless steel is approximately 30 times greater than that of G10. However, stainless steel is also much stronger than G10, as a result of which the ratio of strength to thermal conductivity must be used as a key indicator. The 0.2% yield strength of stainless steel, which would be used for the design, is typically 360 MPa (for 1.4301); the tensile strength of G10 is approximately 270 Mpa.

Even under the conservative assumption that, in the case of G10, a safety factor of 3 is applied with respect to the tensile strength and that stainless steel can be loaded up to the yield strength, the thermal conduction of a stainless steel tube [(270/3 Mpa)/(1 W/cm)]/[(360 Mpa)/(30 W/cm)]=7.5 times as great as a GRP tube having the same load capacity.

Preferably, the supporting structure 4 a made of plastics material will support a metal extension 5 a′, 5 a″, preferably made of stainless steel, at each of its two ends. The metal extensions 5 a′, 5 a″ each have a length of between 20 mm and 100 mm, preferably approximately 50 mm, and a cross-sectional area of stress of between 50 mm² and 500 mm².

As shown in FIG. 1, in embodiments of the invention, the upper end of the inner tube 4 a can be closed in a fluid-tight manner in normal operation, in particular by a pressure relief valve or a rupture disk 9 so that cryogenic fluid flowing away in normal operation has to flow through the gap 4 c between the inner tube 4 a and the outer tube 4 b. The gap 4 c between the inner tube 4 a and the outer tube 4 b comprises a flow restrictor 7 at the end of the neck tube 4 which is closer to the cryogenic tank 3.

FIG. 2 shows an embodiment of the invention which is an alternative thereto, in which, inside the neck tube 4, in particular inside the inner tube 4 a and/or between the supporting structure 4 a and the outer tube 4 b, so-called baffles 6 are installed, which absorb thermal radiation and prevent convection. The baffles 6 are preferably foldable so that, in the case of a rapid flow-off of the cryogen (e.g., in the case of a quench), the baffles release the cross section of the inner tube 4 a and, in this way, restrict the pressure increase in the cryogenic tank 3.

In other embodiments of the invention, the annular gap 4 c can also be used as a pump line for supercooled systems. If the inlet of the pump line is protected by a restrictor, the quench pressure does not have to be taken into consideration when dimensioning the outer tube 4 b. Absolute fluid-tightness between the pump line (annular gap 4 c) and the cryogenic tank 3 (or the volume in the inner tube 4 a) is not necessary. A low leakage flow is acceptable if it is minor by comparison with the flow which is pumped out by the refrigerator.

FIG. 3 shows an embodiment designed in this manner in which the cryostat contains a JT cooler, in which cryogenic fluid is depressurized using a pump located outside the vacuum tank 2 (not shown in the drawings), wherein the gap 4 c between the supporting structure 4 a and the outer tube 4 b is part of the connecting line between the JT cooler and the pump. FIG. 3 shows a cryogenic tank which is divided into two regions using a thermal barrier 20. The thermal barrier 20 is thermally insulating but allows pressure equalization between the two regions (e.g., a flexible membrane made of thermally insulating material). Above the thermal barrier, cryogen is, for example, at atmospheric pressure in the saturation state. Below the barrier, the cryogen is in the supercooled state (e.g., atmospheric pressure, but a temperature which is below the equilibrium temperature). Therefore, heat must be conducted away from the JT cooler. Such an arrangement is ideally suited to the operation of superconducting magnet coils at temperatures below 4.2 K.

In order to avoid mechanical redundancy of the system, a bellows portion 8 can be provided in the outer tube 4 b. The outer tube 4 b thus cannot absorb any axial forces and therefore also cannot be unduly strained by axial forces. An embodiment of the invention which is configured in this way is shown in FIG. 4.

Also conceivable are embodiments of the invention in which, by omitting the exhaust-gas cooling in the annular gap 4 c or the availability as a pump line, a variant without an annular gap is implemented, or in which the supporting structure—unlike as shown in the drawings—is not in the form of a tube, but rather of an individual rod or a plurality of rods.

The features of all the above-described embodiments of the invention can—largely—also be combined with one another. 

What is claimed is:
 1. A cryostat arrangement, comprising a vacuum tank and a cryogenic tank, which is arranged inside the vacuum tank, wherein the vacuum tank comprises at least one neck tube having a supporting structure surrounded by an outer tube, the neck tube leading to the cryogenic tank, wherein the neck tube connects an internal volume of the cryogenic tank to a region outside the vacuum tank so that cryogenic fluid can flow out of the cryogenic tank into a region outside the vacuum tank or from the region outside the vacuum tank into the cryogenic tank, wherein parts of the neck tube used to mechanically suspend the cryogenic tank inside the vacuum tank and parts of the neck tube used to construct a diffusion barrier between an interior of the cryogenic tank and an interior of the vacuum tank are arranged to be spatially separated from one another and are produced from materials which are optimized independently of one another, such that the supporting structure supports a weight of the cryogenic tank, and the outer tube is produced from a material through which the cryogenic fluid cannot diffuse, or through which essentially no cryogenic fluid can diffuse, and wherein the outer tube is configured to connect to other components of the cryostat arrangement in a fluid-tight manner.
 2. The cryostat arrangement according to claim 1, wherein the supporting structure is in a form of an inner tube, and wherein the inner tube connects the internal volume of the cryogenic tank to a region outside the vacuum tank so that the cryogenic fluid from the cryogenic tank can flow into a region outside the vacuum tank or from outside the vacuum tank to flow into a region inside the cryogenic tank.
 3. The cryostat arrangement according to claim 2, wherein the outer tube is in direct contact with the inner tube.
 4. The cryostat arrangement according to claim 2, wherein the outer tube is at a distance from the inner tube, and a gap remains open between the inner tube and the outer tube.
 5. The cryostat arrangement according to claim 4, wherein the outer tube and the inner tube are interconnected by a plurality of axially arranged, radially extending thermal bridges.
 6. The cryostat arrangement according to claim 1, wherein the supporting structure is produced from plastics material that is fiber reinforced.
 7. The cryostat arrangement according to claim 1, wherein the supporting structure made of plastics material comprises a metal extension at each of its two ends.
 8. The cryostat arrangement according to claim 7, wherein the metal extensions each have a length of between 20 mm and 100 mm and a cross-sectional area of stress of between 50 mm² and 500 mm².
 9. The cryostat arrangement according to claim 1, wherein the outer tube is produced from metal.
 10. The cryostat arrangement according to claim 1, wherein inside an inner tube of the neck tube and/or between the supporting structure and the outer tube, baffles are installed, which absorb thermal radiation and prevent convection.
 11. The cryostat arrangement according to claim 11, wherein the baffles are foldable.
 12. The cryostat arrangement according to claim 2, wherein an upper end of the inner tube is closed in a fluid-tight manner in normal operation by a pressure relief valve or a rupture disk, allowing the cryogenic fluid flowing away in normal operation to flow through a gap between the inner tube and the outer tube.
 13. The cryostat arrangement according to claim 1, wherein the cryostat contains a Joule-Thomson (JT) cooler, in which the cryogenic fluid is depressurized using a pump located outside the vacuum tank, and the gap between the supporting structure and the outer tube is part of a connecting line between the JT cooler and the pump.
 14. The cryostat arrangement according to claim 4, wherein the gap between the inner tube and the outer tube comprises a flow restrictor at an end of the neck tube which is near the cryogenic tank.
 15. The cryostat arrangement according to claim 1, wherein the outer tube comprises at least one bellows portion so that the outer tube does not absorb any axial forces.
 16. The cryostat arrangement according to claim 1, wherein the neck tube is produced from a material for which: σ is a maximum permissible mechanical stress, and σ>100 MPa; θ is an integral of the thermal conductivity λ over the temperature range ΔT between 300 K and 4 K, and θ<300 W/m; and wherein a ratio σ/θ>⅓ (MPa·m)/W.
 17. The cryostat arrangement according to claim 1, wherein an integral leakage rate out of the cryogenic tank into the vacuum tank is less than 10⁻⁶ mbar·l/s.
 18. The cryostat arrangement according to claim 2, wherein the outer tube is in thermal contact with the inner tube.
 19. The cryostat arrangement according to claim 6, wherein the fiber-reinforced plastics material is G10.
 20. The cryostat arrangement according to claim 7, wherein the metal extension is made of stainless steel.
 21. The cryostat arrangement according to claim 8, wherein the outer tube is made of stainless steel. 